Knowing the wind velocity of atmospheric air may be of interest for such as meteorological purposes or the effect on heating and cooling of an object. Thus, there is a general continuing need for measuring well the motion of air in the atmosphere. And there is a generalized and continuing need for measuring the motion of fluids transverse to an observation point, for instance, or to determine volume flow of fluids, both within and without conduits in commercial and industrial processes. As a corollary, it is desirable to measure the velocity of a vehicle or other thing relative to the air or gas or other fluid mass through which it moves. For, instance, it may be desired to know the velocity of a motor vehicle or water craft relative to the ambient air mass.
There are many devices known for measuring the velocity of fluids. However, it may be particularly difficult to accurately measure very low velocities, particularly velocities of low mass fluids such as common gases. One problem that may be encountered is that there can be unwanted interaction between the fluid and the means used for measuring velocity. For example, a sensor placed in a gas stream can be subject to erosion, corrosion, accretion, etc. A positive displacement flow meter may be subject to wear. In another aspect, a sensor may disrupt the fluid flow. For example, an orifice plate, propeller or turbine can create undesirable pressure drop and turbulence. In other examples, a variable area or rotometer type flow meter may create pressure drop and have limited range; a heated probe may react with the fluid; a pitot tube may become clogged by debris or condensation, and so forth. Thus it is desirable to have a means which does not require interposition into a fluid stream, in particular a gas stream of a sensor which interacts with the fluid.
Another problem presents when the cross sectional area of the flowing fluid is large, as in a large conduit or free space. The fluid velocity can vary markedly from point to point within the moving fluid mass at the location where it is being measured. To cope with that phenomenon in the prior art an algorithmic calculation may be made based on data taken at one location. For example, the algorithm may assume a certain velocity distribution in the plane of the cross section of a conduit. Thus, accuracy depends on the quality of the algorithm and constancy of fluid behavior. Alternately, a multiplicity of measurements may be made at different locations, as by either using a multiplicity of sensors, or by using a traversing mechanism to move one sensor from point to point. It would be desirable to avoid either alternative and to have a better way of ascertaining the integrated effect of velocity which varies from point to point within the moving fluid mass.
The present invention employs particular lasers. In the prior art, laser Doppler flow meters are useful because they do not require a probe in the gas stream. But commercial Doppler type devices depend for their operation on the presence of particulate matter in the gas. And, particularly when the particles are large, the particle velocity will be less than the velocity of the gas itself, producing probable unknown error.